8.1 Preparing Alkenes: A Preview of Elimination Reactions
2 min read•may 7, 2024
Elimination reactions are key to creating alkenes, those versatile double-bonded hydrocarbons. Two main types are , which removes HX from alkyl halides, and , which removes water from alcohols. Both processes require specific conditions and follow distinct mechanisms.
Understanding these reactions is crucial for predicting alkene products and isomers. helps determine the major product, while considering factors like and reaction mechanisms allows for more accurate predictions. This knowledge is essential for synthesizing alkenes in organic chemistry.
Elimination Reactions to Prepare Alkenes
Types of alkene elimination reactions
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- Dehydrohalogenation removes a hydrogen halide (HX) from an to form an alkene
- Requires a strong base (, )
- General reaction: R-CH2-CH2-X → R-CH=CH2 + HX
- Dehydration eliminates water (H2O) from an to produce an alkene
- Uses a strong acid catalyst (, )
- Carried out at high temperatures (100-200°C)
- General reaction: R-CH2-CH2-OH → R-CH=CH2 + H2O
Dehydrohalogenation and dehydration processes
- Dehydrohalogenation mechanism involves base removing a proton from carbon adjacent to halogen-bearing carbon
- Simultaneous elimination of halide ion forms carbon-carbon double bond
- Example: CH3-CH2-CH2-Br + NaOCH2CH3 → CH3-CH=CH2 + NaBr + HOCH2CH3
- Dehydration mechanism starts with acid protonating the hydroxyl group to create a good (H2O)
- Adjacent carbon loses a proton, forming a carbon-carbon double bond
- Water eliminated as
- Example: CH3-CH2-CH2-OH + H2SO4 → CH3-CH=CH2 + H2O
Predicting alkene products and isomers
- Zaitsev's rule states the most stable (highly substituted) alkene is the major product
- More substituted double bonds are stabilized by
- can produce less substituted alkenes as minor products under certain conditions
- Favored by bulky bases, poor leaving groups, or high temperatures
- Potential isomers include:
- Cis-trans (geometric) isomers differing in spatial orientation of substituents across double bond
- Cis has substituents on same side of double bond
- Trans has substituents on opposite sides
- Constitutional (structural) isomers have same molecular formula but different connectivity
- Example: CH3-CH=CH-CH3 (but-2-ene) and CH2=CH-CH2-CH3 (but-1-ene)
- Cis-trans (geometric) isomers differing in spatial orientation of substituents across double bond
- To predict products:
- Identify the precursor molecule ( or )
- Locate possible elimination sites adjacent to -OH or -X
- Apply Zaitsev's rule to determine major product
- Consider potential isomers based on precursor molecule structure
- Evaluate to determine the position of the double bond formation
Mechanistic considerations in elimination reactions
- : unimolecular elimination involving a intermediate
- Occurs in two steps: slow formation of carbocation, followed by rapid loss of proton
- : bimolecular elimination occurring in a single concerted step
- Base removes proton as leaving group departs simultaneously
- Stereochemistry of elimination:
- E2 reactions often proceed with anti-periplanar geometry
- E1 reactions can lead to a mixture of stereoisomers due to carbocation intermediate
Key Terms to Review (33)
Alcohol: In the context of organic chemistry, an alcohol is an organic compound in which a hydroxyl group (-OH) is bonded to a saturated carbon atom. The general formula for a simple alcohol can be represented as CnH2n+1OH, where n is the number of carbon atoms.
Alcohol: Alcohols are a class of organic compounds characterized by the presence of a hydroxyl (-OH) functional group attached to a saturated carbon atom. They are widely used in various chemical reactions and have diverse applications in organic synthesis, pharmaceutical industry, and everyday life.
Alkyl halide: An alkyl halide is an organic compound in which one or more hydrogen atoms in an alkane (saturated hydrocarbon) have been replaced by a halogen atom (fluorine, chlorine, bromine, or iodine). This substitution results in a molecule with distinct chemical and physical properties compared to its alkane precursor.
Alkyl Halide: An alkyl halide is a type of organic compound that consists of an alkyl group (a hydrocarbon chain) bonded to a halogen atom (fluorine, chlorine, bromine, or iodine). These compounds are important intermediates in many organic reactions, including polar reactions, elimination reactions, and substitution reactions.
Anti stereochemistry: Anti stereochemistry describes the spatial arrangement in a chemical reaction where two substituents are positioned on opposite sides of a double bond or ring structure after the reaction. It is particularly relevant in the halogenation of alkenes, resulting in products where the added atoms are located across from each other.
Carbocation: A carbocation is a positively charged carbon atom that is part of an organic molecule. These reactive intermediates play a crucial role in various organic reactions, including electrophilic additions, nucleophilic substitutions, and elimination reactions.
Chain-growth polymers: Chain-growth polymers are a type of polymerization where monomers add to the growing chain one at a time, with each addition resulting in the immediate formation of the final polymer structure. This process typically involves reactive intermediates such as free radicals or ions.
Cis-trans Isomers: Cis-trans isomers are a type of stereoisomerism that occurs in alkenes, where the substituents on the double-bonded carbons are either on the same side (cis) or on opposite sides (trans) of the double bond. This concept is crucial in understanding the stereochemistry of alkenes and the E/Z designation.
Constitutional isomers: Constitutional isomers are compounds that have the same molecular formula but differ in the sequence in which their atoms are connected. These variations lead to molecules with distinct physical and chemical properties, despite having the same numbers of each type of atom.
Constitutional Isomers: Constitutional isomers are a type of structural isomerism where molecules have the same molecular formula but differ in the connectivity or arrangement of their atoms. This concept is essential in understanding the properties and behavior of organic compounds across various topics in chemistry.
Dehydration: Dehydration is a chemical process in which water is removed from a compound, typically resulting in the formation of a new compound with fewer hydrogen and oxygen atoms. This term is particularly relevant in the context of various organic reactions and transformations, where dehydration plays a crucial role in the preparation and interconversion of different functional groups.
Dehydrohalogenation: Dehydrohalogenation is an elimination reaction in organic chemistry where a hydrogen atom and a halogen atom (such as chlorine, bromine, or iodine) are removed from an organic compound, resulting in the formation of an alkene or alkyne. This process is a key step in the preparation of unsaturated hydrocarbons from alkyl halides.
E1 reaction: An E1 reaction is a type of elimination reaction in organic chemistry where a substrate, typically an alkyl halide, undergoes deprotonation to form an alkene. This process occurs in two steps, involving the formation of a carbocation intermediate followed by the loss of a proton.
E1 Reaction: The E1 reaction, or unimolecular elimination reaction, is a type of organic chemistry reaction in which a leaving group is removed from a substrate, resulting in the formation of an alkene. This process occurs in a stepwise manner, involving the formation of a carbocation intermediate.
E2 reaction: An E2 reaction is a bimolecular elimination reaction where a hydrogen atom is removed from a carbon adjacent to the one bonded to the leaving group, resulting in the formation of an alkene. This process involves a single concerted step, where both the base removing the hydrogen and the leaving group departure occur simultaneously.
E2 Reaction: The E2 reaction is an elimination reaction in organic chemistry where a base removes a hydrogen atom and a leaving group from adjacent carbon atoms, resulting in the formation of an alkene. This reaction is characterized by the simultaneous removal of the hydrogen and the leaving group, proceeding through a concerted mechanism.
Elimination Reaction: An elimination reaction is a type of organic reaction in which two atoms or groups are removed from a molecule, typically resulting in the formation of a carbon-carbon double bond or a carbon-carbon triple bond. This process is an important step in the synthesis of alkenes and alkynes, as well as in various other organic transformations.
Geometric Isomers: Geometric isomers, also known as cis-trans isomers, are a type of stereoisomerism that occurs when two identical substituents are arranged on the same side (cis) or opposite sides (trans) of a carbon-carbon double bond. This structural feature has important implications for the physical and chemical properties of organic compounds.
Hofmann Elimination: The Hofmann elimination is a type of elimination reaction that involves the removal of a hydrogen atom and a leaving group from a quaternary ammonium salt, resulting in the formation of an alkene. This reaction is particularly important in the context of preparing alkenes, understanding the structure and properties of amines, and the reactions of amines.
Hofmann elimination reaction: The Hofmann elimination reaction is a chemical process where an amine is converted into an alkene through the treatment with excess methyl iodide, followed by silver oxide (Ag2O), and then heating with water. This reaction involves the removal of a hydrogen atom and a leaving group from adjacent carbon atoms in a substrate, leading to the formation of a double bond.
Hyperconjugation: Hyperconjugation is a type of conjugation in organic chemistry where the sigma bonds of alkyl groups (such as methyl or ethyl) interact with adjacent pi bonds, leading to increased stability of the molecule. This stabilizing effect is particularly important in understanding the stability of carbocations and the orientation of electrophilic additions.
Leaving group: A leaving group in organic chemistry is an atom or group that detaches from the parent molecule during a nucleophilic substitution (SN2) reaction, forming a lone pair or negative ion. The ease with which a leaving group departs affects the rate and success of the reaction.
Leaving Group: A leaving group is a functional group or atom that is displaced or removed from a molecule during a chemical reaction. It is a key component in many organic reactions, particularly substitution and elimination reactions, as it facilitates the formation of a new bond or the creation of a new product.
Phosphoric Acid: Phosphoric acid is a colorless, odorless, and highly corrosive inorganic compound with the chemical formula H3PO4. It is a key player in various chemical reactions, particularly in the context of electrophilic addition reactions of alkenes and the preparation of alkenes through elimination reactions.
Phosphoric acid anhydride: Phosphoric acid anhydride refers to a compound that results from the dehydration of phosphoric acid, leading to a more reactive form often involved in biochemical energy transfer. It plays a critical role in synthesizing ATP, the energy currency of cells, by forming high-energy phosphate bonds.
Potassium tert-butoxide: Potassium tert-butoxide is a strong base used in organic chemistry, particularly in elimination reactions to prepare alkenes and alkynes. It is a white, crystalline solid that is highly reactive and must be handled with care.
Regioselectivity: Regioselectivity refers to the preference of a chemical reaction to occur at a specific site or region of a molecule, leading to the formation of one regioisomeric product over another. This concept is particularly important in the context of electrophilic addition reactions of alkenes, electrophilic aromatic substitution, and other organic transformations.
Sodium Ethoxide: Sodium ethoxide is an alkoxide compound with the chemical formula C₂H₅ONa. It is a strong nucleophile and base used in various organic reactions, including the preparation of alkenes, the Wittig reaction, and Claisen condensations.
Stereochemistry: Stereochemistry is the study of the three-dimensional arrangement of atoms in molecules and how this arrangement affects the chemical and physical properties of the substance. It examines the spatial orientation of atoms and their relationship to one another, which is crucial in understanding many organic chemistry concepts.
Structural Isomers: Structural isomers are molecules that have the same molecular formula, but different arrangements of atoms in space. This results in distinct chemical and physical properties, despite the identical elemental composition.
Sulfuric Acid: Sulfuric acid (H2SO4) is a highly corrosive, dense, and oily liquid that is one of the most important and widely used industrial chemicals. It is a strong mineral acid that plays a crucial role in various chemical reactions and processes.
Vinyl monomer: A vinyl monomer is an organic compound containing at least one vinyl group (-CH=CH2) that can undergo polymerization to form polyvinyl compounds. These monomers are crucial in producing a wide range of plastic materials through addition reactions.
Zaitsev's Rule: Zaitsev's rule is a principle in organic chemistry that predicts the major product of an elimination reaction. It states that the major alkene product will be the one with the most substituted (most stable) double bond.